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Originally the plan had been to replace the bridge, but that idea was scrapped when planners realized it would take an extra $1 million and another year of construction to remove all of the electrical wires associated with Metrorail.

Mr. Shakeri said the plan was modified to focus on the bridge’s substructure, “make it better than the original,” and build a new superstructure using existing parts of the bridge to help lower costs and construction time.

The superstructure, Mr. Shakeri said, is “what’s underneath your tires.”

Full-scale construction began in April 2011, and workers are on the job 24 hours a day. At any given time, between 20 and 25 people are working on the bridge. They regularly pause their work to allow trains to go by.

“It’s like building a house working only a few hours a day,” Mr. Shakeri said.